2 INTRODUCTOEY. 



powerful influence, by allowing an immediate application to geological problems of 

 such general laws as are established beyond any doubt. Besides, the improvement of 

 the practical methods applied in chemical and physical laboratories has given rise to 

 the execution of numerous experiments, which were made with a view of imitating the 

 processes applied in the great natural laboratories. They suggested frequently the 

 causes of facts disclosed by the examination of a certain region, or obtained by com- 

 paring the observations made in different countries on one certain subject. 



It appears to be mainly due to these causes that a number of theories have 

 been proposed, in rapid succession, relating to the origin of rocks, to the mode and causes 

 of metamorphism, to the agencies of vulcanism, to the structure and mode of formation 

 of mountain ranges, to the structure of the entire globe, and other cognate subjects. 

 It will be admitted, even by those who are most strongly opposed to theorizing, that 

 geological science has in this way been promoted and enriched in various respects ; 

 since there is scarcely a theory which, even if insufficient to explain what it under- 

 takes, has not some truth in it, or is applicable to some extent in certain cases, or has 

 at least, even when proving to be erroneous itself, led to discussions on subjects of high 

 interest, which is indeed a result of no little value. In reviewing the theories pro- 

 posed on any particular subject, we find them, it is true, often in apparent contradic- 

 tion with each other ; yet almost every one is based upon arguments drawn from 

 observed facts, and there are probably very few which will ultimately be entirely 

 abandoned. The immense range of varied processes as applied by nature allows the 

 applicability, in a limited way, of many a theory in certain instances, while in others 

 it may be refuted on no less valid grounds ; and the struggle between the defenders of 

 different doctrines is often founded only in the difference of their standing-points. 

 What appears to be true in one instance is frequently not applicable in others ; it is 

 the bold generalizations which render theories so often untenable in that form in which 

 they are usually first expressed. An instructive example is presented by the different 

 theories which have been proposed for explaining the mode of formation of mineral 

 veins. Almost every one of them was based upon a limited range of observations, and 

 was, from its first application to a few instances, extended to the generality of veins. 

 Numerous exceptions to it were then found, leading to the rejection of the first, and 

 the establishment of a different, theory, which, in its turn, shared a similar fate. Ob- 

 scure as this subject still is, we are able to state this as certain, with our present state 

 of knowledge, that every mineral vein is the product, not of one simple but of complex 

 processes. Nearly every one of the theories proposed will, therefore, have its limited 

 range of applicability, inasmuch as the agent it suggests may have been especially 

 active in the formation of the veins of a certain order, while the same agent may have 

 played a subordinate part in regard to the origin of other veins which were chiefly 

 due to processes of another kind. 



This instance points clearly towards the one principal cause of the divergence 

 of opinions in regard to some of the most important geological questions. This cause 

 is the want of latitude of the basis upon which arguments are founded. Conclusions 

 which are obtained by reasoning on geological subjects solely on the strength of chem- 

 ical analysis, are, when generalized, often found to be utterly in discordance with the 

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